Consider running terraform validate against supplied configuration

### terraform-plugin-testing version

```
v1.6.0
```

### Use cases

`TestStep.Config` uses strings to represent terraform configuration. There is no guarantee that the configuration that is supplied is valid. TestStep.ConfigDirectory`, and TestStep.ConfigFile` provide for supplying terraform configuration in either a directory, or a file, respectively, but do not validate the configuration before it is executed.

### Proposal

The terraform configuration supplied through `Config`, `ConfigDirectory`, or `ConfigFile` could be validate using `terraform validate` prior to execution, and errors raised if the configuration is invalid.

### References

- #264